Various electrically-powered equipment within an industrial environment often depend upon any of a variety of AC and/or DC voltages for power. More specifically, DC-oriented systems tend to utilize relatively low DC voltages, typically ranging from 12 to 50 volts DC (VDC). AC-oriented systems, however, often employ higher AC voltages, sometimes ranging between 100 and 250 volts root-mean-square (VRMS). Other AC or DC voltages outside of these ranges may be employed as well. However, industrial instrumentation, such as a Coriolis flowmeter for measuring mass flow and other information concerning a material flowing through a conduit, often employ electrical components that require a low DC voltage, such as 1.2-24 VDC, as an electrical power source, and thus are not capable of withstanding such a large range of AC or DC input voltages. Thus, a step-down converter capable of producing a substantially fixed low DC output voltage from either an AC or DC input voltage is often used to great advantage in such an environment.
A simplified schematic diagram of one particular type of step-down, or “buck,” converter or regulator 1 currently in use for converting a positive DC input voltage VIN to a DC output voltage VOUT is provided in FIG. 1. The input voltage VIN is asserted across an input capacitor CA coupled with a ground reference, and is coupled with the drain terminal of an n-channel power field-effect transistor (FET) switch Q. The input capacitor CA acts as a filter to help maintain the voltage level seen by the drain of the switch Q in the presence of changes in the input voltage VIN by providing additional current on a temporary basis to the drain of the switch Q. Similar functionality for the output voltage VOUT is provided by an output capacitor CB.
The gate of the switch Q is driven by a switch controller 2, which turns the switch Q ON and OFF depending on the voltage level of the output voltage VOUT compared to the desired or target output voltage VOUT level. Some other measurable quantity at the output, such as current, may be employed by the switch controller 2 alternatively or additionally. By turning the switch Q ON and OFF substantially periodically, the switch controller 2 is normally capable of maintaining the output voltage VOUT at a desirable level in the presence of changes in both the input voltage VIN level and the load driven by the output voltage VOUT. Generally, the switching period is the sum of the ON time and the OFF time of the switch Q during one operation cycle. Accordingly, the duty cycle of the switch Q is the ratio of the ON time to the period. Thus, by way of any of a number of techniques, the switch controller 2 controls the duty cycle and period of the switch Q to maintain the output voltage VOUT at a satisfactory level.
During operation of the converter 1, when the switch Q is ON, electrical current flows from the input voltage VIN, through the drain and source terminals of the switch Q, and through an inductor L, to the output voltage VOUT. As a result of electrical current flowing through the inductor L, electrical energy is stored in the inductor L. Typically, the ON time of the switch Q, as set by the switch controller 2, is constrained by the component values of the inductor L and the output capacitor CB such that the voltage VL across the inductor is nearly constant during the ON time. Under these conditions, the terminal of the inductor L connected to the source terminal of the switch Q remains near the input voltage VIN while the switch Q is ON, and the remaining terminal of the inductor L is at the output voltage VOUT level. As a result, the voltage at the cathode 3 of a diode D coupled at the source terminal of the switch Q causes the diode D to be reverse-biased, and therefore not conducting, when the switch Q is ON, since the anode 4 of the diode D is connected to ground.
When the switch Q is then turned OFF, the voltage VL across the inductor L reverses polarity in order to maintain the continuity of the electrical current through the inductor L. That “flyback” in voltage causes the voltage at the cathode 3 of the diode D to drop below ground, thereby forward-biasing the diode D into conduction. Thus, electrical energy stored in the inductor L while the switch Q is ON is transferred as current through the diode D and the inductor. L to the output voltage VOUT. At some point determined by the switch controller 2, the switch Q is once again turned ON, and the above cycle repeats. Current thus flows into the voltage output VOUT when the switch Q is either ON or OFF.
One potential drawback of the step-down converter 1 of FIG. 1 is the large voltage swing required from the switch controller 2 to drive the gate of the switch Q to turn the switch Q ON and OFF. More specifically, to turn the switch Q ON and maintain that state, the switch controller 2 must drive the gate to a voltage level higher than the input voltage VIN, since the gate voltage must be above that of the source, which is close to the input voltage VIN during the ON state. To turn the switch Q OFF, the gate voltage must be near ground, since the source is driven to slightly below ground due to the diode D becoming forward-biased at that time due to the flyback of the inductor L. When the input voltage VIN is a relatively low DC voltage, generation of the proper gate voltage for the switch Q to be turned ON may be accomplished by way of a readily-available voltage “boost” circuit. However, when the input voltage VIN is a large AC voltage on the order of 265 VRMS, which translates to a maximum DC voltage level of about 375 VDC, timely and accurate control of the gate voltage while providing extremely large voltage swings at the gate of hundreds of volts typically requires a relatively complex circuit design for the switch controller 2 involving specialized electrical components.